When migrating to an IP-based telephony solution, making sure you have a way to support legacy analog requirements is critical to your success. The most common analog requirements of course, are fax machines and users that need a modem for some legacy application. However, there are quite a lot of other analog gotchas, so you'll want to make sure your system is compatible and has available ports before deploying it.
A popular option these days is a monitored security system. Many homes and businesses outsource this security component by connecting an analog line, which is often a 'dry pair' (i.e. no phone service from the telco, just the physical connection) to an external company that monitors alarms and calls the police if necessary. Fire alarms may also be included in this group. The manner in which you connect your fire alarms to the rest of the world may also be regulated by local fire codes, so watch out for this as well.
Another easy system to overlook is an elevator with the ability to call home. Many other large appliances, like copy machines, HVAC equipment and power supplies have this ability as well. Some administrators also like analog lines and a modem to connect to their servers as a backup in case of some network issue.
Since a lot of these analog lines represent security risks to your network as well, it's a good idea to have a comprehensive list regardless of any IP Telephony projects. Between reports from your PBX and telco, you should be able to know exactly how many lines you have. Tracing the cables to find what's using them and who is responsible for it is sometimes more challenging.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.